Use of Psinsar™ Data to Infer Active Tectonics: Clues on the Differential Uplift Across the Giudicarie Belt (Central-Eastern Alps, Italy)

Tectonophysics 476 (2009) 297–303

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Tectonophysics

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Use of PSInSAR™ data to infer active tectonics: Clues on the differential uplift across the Giudicarie belt (Central-Eastern Alps, Italy)

M. Massironi a,⁎, D. Zampieri a, M. Bianchi b, A. Schiavo c, A. Franceschini d a Dipartimento di Geoscienze dell'Università di Padova, Via Giotto 1, 35137 Padova, Italy b Tele-Rilevamento Europa - T.R.E. s.r.l., Via Vittoria Colonna, 7, 20149 Milano, Italy c Land Technology and Services s.r.l., Via Sartorio, 12, 31100 Treviso, Italy d Servizio Geologico della Provincia Autonoma di Trento, Via Roma 50, 38100 Trento, Italy article info abstract

Article history: The Permanent Scatterers Synthetic Aperture Radar INterferometry (PSInSAR™) methodology provides high- Received 21 December 2007 resolution assessment of surface deformations (precision ranging from 0.8 to 0.1 mm/year) over long periods Accepted 28 May 2009 of observation. Hence, it is particularly suitable to analyze surface motion over wide regions associated to a Available online 6 June 2009 weak tectonic activity. For this reason we have adopted the PSInSAR technique to study regional movement across the Giudicarie belt, a NNE-trending trust belt oblique to the Southern Alpine chain and presently Keywords: characterized by a low to moderate seismicity. Over 11,000 PS velocities along the satellite Line Of Sight (LOS) PSInSAR were calculated using images acquired in descending orbit during the 1992–1996 time span. The PSInSAR Active tectonics – Giudicarie belt data show a differential uplift of around 1.4 1.7 mm/year across the most external WNW-dipping thrusts of the Giudicarie belt (Mt. Baldo, Mt. Stivo and Mt. Grattacul thrusts alignment). This corresponds to a horizontal contraction across the external part of the Giudicarie belt of about 1.3–1.5 mm/year. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Radar INterferometry (PSInSAR™)(Ferretti et al., 2000, 2001; Colesanti et al., 2003), allows the identiﬁcation and use, during the interferometric Since the 1993 work of Massonet et al. dealing with the co-seismic process, of radar phase stable points normally corresponding to buildings ground movements related to the Landers earthquake (California, 28 or single outcrops (i.e. permanent scatterers). This strongly minimizes June 1992, Mw 7.3; e.g. Kanamori et al., 1992; Sieh et al., 1993), the phase dispersions and the consequent loss of coherence between time remote sensing Synthetic Aperture Radar INterferometry (InSAR) has delayed acquisitions. Hence, although this method does not give regional been extensively used for active deformation assessment. InSAR uniformly distributed information on surface movements as the classic provides a one-dimensional measurement of change in distance InSAR procedure, it is particularly suitable for high-resolution assess- between the surface and the spacecraft along the Line Of Sight (LOS or ment of surface deformations (precision up to 0.1 mm/year) to be slant-range) of the Synthetic Aperture Radar (SAR) sensor. In the ﬁeld of realized over a long period of observation using a great amount of SAR fault tectonics InSAR processing has been extensively applied to record images. For these reasons PSInSAR has recently proved to be very co-seismic (e.g. Zebker et al., 1994; Massonet et al., 1994; Tobita et al., effective in resolving surface motion of seismic areas during inter-seismic 1998; Pedersen et al., 2003; Wang et al., 2006) and post-seismic (e.g. periods or even of wide regions presently involved by very low tectonic Peltzer et al., 1996; Atzori et al., 2008) signatures, derive focal movements (provided that a detection of a considerable amount of PS is mechanisms, fault geometry and in general source models (e.g. Feigl guaranteed) (Bürgmann et al., 2006; Vilardo et al., 2009). et al.,1995; Acoglu et al., 2006; Peyret et al., 2008; Sudhaus and Jonsson, In this work we have applied the PSInSAR method to obtain new 2009) and even evaluate inter-seismic strain (Cavaliè et al., 2008). insights into the present-day deformation pattern of the southern However, the major draw-back of the classical InSAR techniques, Giudicarie belt, which is a key sector of great tectonic complexity within which rely on a stack of several interferograms, is the possible lack of the post-collisional south-vergent thrust system of the Southern Alps coherence between the time delayed SAR couples acquired on vegetated (Italy) (Castellarin et al., 2006; Massironi et al., 2006; Viganò et al., 2008). and/or mountain areas and/or in different atmospheric conditions. A recent InSAR technique, the Permanent Scatterers Synthetic Aperture 2. The Giudicarie belt

⁎ Corresponding author. Fax: +39 049 8272010. 2.1. Geological setting E-mail addresses: [email protected] (M. Massironi), [email protected] (D. Zampieri), [email protected] (M. Bianchi), [email protected] (A. Schiavo), [email protected] The post-collisional south-vergent belt of the Southern Alps shows (A. Franceschini). its maximum complexity at the Giudicarie belt. NNE-trending system

Fig. 1. Structural sketch of the central and eastern Southern Alps. is oblique to the Southern Alps chain (Fig. 1) and results from tectonic worthy to be mentioned is the southern Mt. Baldo, an area where reactivation and inversion of an Early Jurassic extensional domain several historical earthquakes are reported (Boschi et al., 2000). The related to the continental rifting of the Adria margin (e.g., Bertotti et ridge of the Mt. Baldo is displaced by a 4.5-km-long normal fault, al., 1993). On the whole, the southern Giudicarie belt develops along which originated the narrow Naole valley parallel to the ridge. the transition between basinal (Lombardy basin) and platform Trenches excavated at the base of the fresh carbonate bedrock scarp (Venetian platform) domains (Trevisan, 1939; Castellarin, 1972), affecting the western slope of the Naole valley have shown an activity where many sub-basin depocentres have formed. These basins are of the fault between the Late Pleistocene and Holocene (subsequent to bounded by steep to middle angle synsedimentary faults, generally 17,435–16,385 BP, 14C cal. age, Galadini et al., 2001). A mixed dipping towards the west and E–W steep transfer faults (Castellarin gravitational-tectonic origin for this deformation has been suggested and Picotti, 1990). by Galadini et al. (2001) and Sauro and Zampieri (2001). The Giudicarie fault system exhibits a structural boundary also at crustal (Scarascia and Cassinis, 1997) and lithospheric (Lippitsch et al., 2.2. Seismotectonic setting 2003) scales. In its southern sector (south of the intersection with the Tonale The Giudicarie belt is characterized by low instrumental seismicity fault, Fig. 1) the Giudicarie fault separates the Tertiary Adamello (Slejko et al., 1989; Carulli and Slejko, 2009)withfewmoderate batholith and the Variscan basement to the west and northwest from earthquakes (Pondrelli et al., 2007), most of which are located in the the sedimentary cover of the Southern Alps with thin basement slices upper crust (Scarascia and Cassinis, 1997; Viganò et al., 2008). The most to the east. The South Giudicarie fault system is composed of WNW- important historical earthquakes (I0 IX MCS) for northern Italy occurred dipping steep to subvertical fault planes, which being oblique to the in 1117 in the Verona area and in 1222 near Brescia (Guidoboni, 1986; Alpine belt have accommodated transpression. Its Neogene kinematic Guidoboni and Comastri, 2005; Guidoboni et al., 2005; Stucchi et al., role is that of an oblique ramp of the S-vergent Val Trompia thrust to 2008), both located just at the southern end of the belt. Along the Adige which it is connected towards the south (Picotti et al., 1995)(Fig. 1). valley, the Roman archaeological site of Egna shows evidences of Slip partitioning has produced distributed sinistral strike-slip move- destruction related to an earthquake of similar intensity that happened ments on steep faults and dip-slip reverse movements on moderate in the mid 3rd century AD (Galadini and Galli,1999; Galadini et al., 2001; dipping N to NNE-trending faults within a wide belt comprised Stucchi et al., 2008). In addition, Quaternary faults have been recently between the Giudicarie and Adige valleys (Prosser, 1998; Viola et al., located within the epicentral area of the strong 1222 Brescia earthquake 2001; Massironi et al., 2006). (Sileo et al., 2007; Livio et al., 2009a,b). The study area has been affected by strong exogenic modelling The strongest recent events are the Mw 4.8 Merano earthquake, related to the Late Pleistocene glacial activity and post-glacial ﬂuvial which occurred on 17 July 2001 close to the North Giudicarie Line erosion. Therefore, most geomorphic evidence of Quaternary tectonics (Caporali et al., 2005; Pondrelli et al., 2006) and the ML 5.3 Salò event, has been strongly modiﬁed and often completely erased, and the which struck the western coast of the Lake Garda on 24 November depositional units suitable for dating are quite rare. The only case 2004 (Baer et al., 2005; Pondrelli et al., 2006). M. Massironi et al. / Tectonophysics 476 (2009) 297–303 299

In the seismic source model for seismic hazard assessment (Meletti Satellite INterferometry of Synthetic Aperture Radar data (InSAR) is et al., 2008) this region has been included in a common zone where the based upon phase comparison of SAR images acquired at different times NW-trending strike-slip faults of the Schio-Vicenza fault system and and views (Curlander and McDonough,1991). The Permanent Scatterers the NNE-trending Giudicarie transpressional system interact. In fact, are high RADAR reflecting objects that do not change their spectral the distribution of the events which occurred during the last decades is response for different acquisitions (i.e. mainly edifices and other clustered around the junction between the Giudicarie and the Schio- anthropic objects affected by corner effects, and natural exposed Vicenza systems, just north of the apex of the triangular-shaped Lessini polished surfaces). For this reason the phase dispersion due to the block. The calculation of focal mechanisms from relocated events has temporal and geometrical decorrelation phenomena, that negatively permitted Viganò et al. (2008) to demonstrate that in the Giudicarie affect RADAR interferometry, is strongly minimized using PS as belt the stress regime is consistent with oblique-reverse fault measurement points. The final result is an accurate measurement of kinematics of the NNE-trending faults and the horizontal compressive the movements along the SAR Line Of Sight (LOS velocities) of each PS stress axis is NW-trending. This regime is quite different with respect with respect to an assumed stable point (Reference point or REF) during to the strike-slip regime of the neighbouring Lessini area, where the the time span considered. Since the LOS of a descending orbit is west direction of the maximum horizontal compressive stress axis is directed and only slightly off-nadir looking (about 23°), the PS approximately N–S(Viganò et al., 2008). movements can be fairly related to the PS uplift (Bürgmann et al., 2000). The surface elevation changes along the LOS, in terms of differential displacement between two PS, can be determined with a 3. The PSInSAR™ data precision ranging from 0.1 to 1 mm/year depending on the PS coherence (Colesanti et al., 2003; Ferretti et al., 2007). The PS coherence is a Satellite interpherometric data, provided by the Provincia Autonoma normalized index of the local signal to noise ratio of the interferometric di Trento (PAT), allowed the active differential uplift among different phase (Hanseen, 2001; Colesanti et al., 2003). The data set here crustal blocks across the Giudicarie belt to be assessed. These data were considered was calculated using a REF point at Trento town centre obtained through the “Permanent Scatterers (PS) Technique” (Ferretti (Gauss–Boaga coordinates: north: 5103161.50; east: 1664454.75) and et al., 2000, 2001) using a temporal series of 80 ERS1 and ERS2 images consists of 11,038 points (Fig. 2) with a coherence higher than 0.56 acquired in descending orbits and in a period covering the 1992–1996 which means a PS annual mean velocity estimates within 0.8 mm/year time span (Track 165, Frames 2673 and 2691). of accuracy (2003 data set from Tele-Rilevamento Europa - T.R.E. s.r.l.).

Fig. 2. PSInSAR data of the Adige (a) and Sarca (b) valleys. Blue dots: Uplifting Permanent Scatterers (N0.8 mm/year); green dots: stationary Permanent Scatterers (0.8≤velocity≥−0.8 mm/year); red dots: subsiding Permanent Scatterers (b−0.8 mm/year). A–A' is the trace of the cross-section of Fig. 4. The PS–LOS velocities within the 1000 m wide grey buffer were plotted in Fig. 4. The star is the Trento Reference point (REF). 300 M. Massironi et al. / Tectonophysics 476 (2009) 297–303

Fig. 3. Histograms of the PSInSAR velocities distribution in the Adige (a) and Sarca (b) valley areas as deﬁned in Fig. 2.

The use of PSInSAR measurements for our regional tectonic study approximate vertical strains, the true vertical and horizontal compo- of a mountain area affected by a moderate tectonic activity must take nents can be estimated only with a data set acquired in both ascending into account the following points: i) the limited time of observation and descending orbits (Vilardo et al., 2009) or, alternatively, GPS- (1992–1996); ii) the PS distribution is not uniform since PS are derived horizontal velocity ﬁeld can be used to eliminate its preferentially concentrated in towns, villages and along main valleys, contribution on the PSInSAR measurements and consequently obtain while they are almost lacking on the most relieved areas because they the true vertical component (Burgmann et al., 2006; Ferretti et al., are less inhabited, vegetated and strongly affected by foreshortening, 2007). Both these procedures can not be applied in our case since only layover and shadowing effects on the related SAR images; iii) the a descending orbit data set is available and the region lacks a dense estimations of the PS annual velocities are very accurate in the range GPS network. of 1 km from the REF (precision around 0.1 mm/year) but decrease in For these reasons the PSInSAR data were evaluated in a map view accuracy when larger distances are considered (in our case up to considering PS with LOS velocities between −0.8 mm/year and 0.8 mm/year); iv) the velocity outliers (located outside 2 standard +0.8 mm/year as stable points, PS with LOS velocities N0.8 mm/year deviation of the LOS velocity distribution and reaching up to +6 mm/ as uplifting points and PS with LOS velocities b−0.8 mm/year as year and −20 mm/year), are not related to tectonics but to landslides, subsiding points (Fig. 2). Hence, the PS LOS velocities distribution has quarries, excavations and urban growth (see for example blue PS east been plotted for two wide and representative areas: one at the and north of Trento, Fig. 2); v) although the LOS velocities fairly footwall of the Giudicarie belt frontal thrusts (Adige valley, area “a” in

Fig. 4. Schematic cross-section of the frontal part of the Giudicarie belt (trace in Fig. 2; modiﬁed from Castellarin et al., 2005) and related PS–LOS velocities. Mean PS–LOS velocities of three subgroups across the Mt. Stivo thrust system show an uplift decrease from the west (hanging wall, sector “b” of Fig. 2) to the east (footwall, sector “a” of Fig. 2). M. Massironi et al. / Tectonophysics 476 (2009) 297–303 301

Figs. 2 and 3) and the other one at the hanging wall (Sarca valley, area crustal block at the hanging wall of the Giudicarie belt frontal thrusts “b” in Figs. 2 and 3). In addition, LOS velocities have been plotted also and a sector to the east showing velocities comparable to the REF. against a key geological section crossing the Giudicarie belt in order to In Fig. 4 LOS velocities are plotted against a geological section derived quantitatively estimate the motions across its most external thrusts from Castellarin et al. (2005) and showing one of the major external (Fig. 4). In this case the plotted LOS velocities derive from a 1 km wide thrust of the Giudicarie belt, the Mt. Stivo thrust system. The differential corridor containing the geological section and were projected uplift across this system actually represents the relative motion between perpendicularly to the section (Fig. 2). Sarca and Adige valleys. Thanks to the low off-nadir angle of looking direction (around 23°) LOS velocities roughly approximate vertical motions, hence according to Figs. 2 and 4 the Mt. Stivo thrusts and/or the 4. Results and discussion anticlinalic fault propagation folds at their hanging wall should accommodate a total amount of around 1.4–1.7 mm/year of differential The results show a wide and unequivocally uplifting area along the uplift (mean LOS velocity at the hanging wall=+1.31 mm/year vs mean lower Sarca valley west of the Giudicarie belt frontal thrusts (i.e. Mt. Baldo, LOS velocity at the footwall=−0.38 mm/year along the cross-section of Mt. Stivo and Mt. Grattacul thrusts alignment) (Fig. 2). This area (“b” in Fig. 4 or mean LOS velocity of area “b”=+1.09 mm/year vs mean LOS Fig. 2) is strongly dominated by uplifting PS, whereas subsiding points are velocity of area “a”=−0.31 mm/year in Fig. 2). In more detail, this almost lacking or are due to local effects (landslides, quarries, and differential uplift seems to be mainly due to the activity of the excavations). In this case on a total amount of 24,194 PS, only 134 have LOS westernmost thrust fault of the system (Fig. 4). From the geological velocities b−0.8 mm/year, whereas the uplifting PS are 16,940 rising to map and sections of Castellarin et al. (2005) the Mt. Stivo thrusts trend 23,021 if LOS velocities N+0.1 mm/year are considered. By contrast, uplift N44E and dip toward the NW with an angle of 48°. On the basis of the is almost absent along the Adige valley between Rovereto and Trento, regional stress tensor calculated by Viganò et al. (2008) for the Giudicarie where subsiding and stationary PS prevail. This can be quantitatively belt (principal stress axes: σ1 =142/30, σ2 =247/24, σ3 =009/50; R= evaluated comparing the mean LOS velocities value of the Sarca valley (σ2−σ1)/(σ3 −σ1)=0.8), it is possible to derive the tangential shear data set (area “b” in Fig. 3:24,194points)whichis1.09mm/yearwitha resolved on the Mt. Stivo thrusts surface and, consequently, the related slip sigma of 0.59, with the LOS velocities to the east of the Giudicarie belt vector (Fig. 5). This trends N355 with a plunge of 39° showing a nowadays (Adige valley area “a” in Fig. 3: 53,432 points) that are distributed around a transpressional activity of this structure (pitch=301°). On the basis of the mean of −0.31 mm/year with a standard deviation of 0.99 (Fig. 3). tangential shear orientation and the relative uplift across the Mt. Stivo Since the velocities considered are referred to a REF point at Trento thrust system, the movement along the slip vector of all the block at its town, the PSInSAR data record a still active motion between the uplifting hanging wall (i.e. the Sarca valley area) can be estimated around 2.2–

Fig. 5. Sketch of the Mt. Stivo fault plane parameters and kinematics derived from the regional stress ﬁeld (Viganò et al., 2008) and PSInSAR LOS velocities. 302 M. Massironi et al. / Tectonophysics 476 (2009) 297–303

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